Right now across the country, tens of millions of older adults and people with serious disabilities have a choice to make: whether to stick with their current Medicare option, or change during Open Enrollment. One of the biggest decisions they face is whether to go with a Medicare Advantage plan offered by an insurance company, or traditional Medicare coverage offered directly by the federal government. If they change from one to the other, a new University of Michigan study finds, they may be entering a revolving door and find themselves changing again in the future.

On average, the study shows, 3% of people with traditional Medicare switch over to an MA plan each year. But when the researchers looked closely at these "switchers", they found that switching to MA was more than twice as common – 6.5% -- among those who had switched from MA to traditional Medicare only one to three years before.

In fact, 9% of those who had jumped from MA to traditional Medicare the year before then switched back to Medicare Advantage, staying just one year in the traditional program. For those who had left MA for traditional Medicare two or three years earlier, the percentage switching back to MA was still much higher than average. Medicare Advantage plans now cover more than half of all Americans with Medicare, so really understanding patterns like these is important from a policy perspective.

What we found here is that it's just not an open-and-shut case that everyone who leaves Medicare A.